Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems for measuring electromagnetic (EM) fields and, more particularly, to mechanisms and techniques for measuring both natural and controlled source electromagnetic fields.
Discussion of the Background
EM surveying is a method of geophysical exploration to determine the properties of a portion of the earth's subsurface, information that is especially helpful in the oil and gas industry and the mining industry. EM surveys may be based on a controlled source that sends EM energy waves into the earth, which induces eddy currents in the earth. The eddy currents generate a secondary EM field or ground response. By measuring the secondary field with an EM receiver, it is possible to estimate the depth and/or composition of the subsurface features. These features may be associated with subterranean hydrocarbon deposits.
A schematic airborne EM survey system 100 generally includes, as illustrated in FIG. 1, a transmitter 102 for generating a primary electromagnetic field 104 that is directed toward the earth. When primary EM field 104 enters the ground 108, it induces eddy currents 106 inside the earth. These eddy currents 106 generate a secondary electromagnetic field or ground response 110. An EM receiver 112 then measures the response 110 of the ground. Transmitter 102 and receiver 112 may be connected to an aircraft 114 so that a large area of the ground is swept. Receiver 112 may be located concentric and/or coplanar with transmitter 102. For a frequency-domain EM (FDEM) sensor, a bucking coil 113 may be added, that is concentric and/or coplanar with the receiver 112.
FDEM systems have been used successfully in the past to map near-surface conductivity structures in the range of 0 to 150 m below the surface. These systems work for frequencies ranging from 400 Hz to 150 kHz. However, a general limitation of these systems is the reduced earth penetration (i.e., up to 150 m). In the seismic field, for example, there are many situations when the oil reserves are below 150 m. Thus, there is a need to use another source and/or system for generating EM fields having lower frequencies, for example, in the range of 10 to 500 Hz, so that a depth of exploration is extended to potentially several kilometers. Note that the lower the frequency, the larger the penetration depth.
Such a source already exists and is associated with natural EM fields that exist in the Earth. Natural magnetic fields are used herein as meaning any magnetic field that is generated by the Earth itself, without human intervention. This term is in a sense opposite to controlled magnetic field, which is a magnetic field generated by human intervention, e.g., with a coil in which a varying current is flowing. Natural EM fields have been used in the past to investigate the conductivity structure of the earth, in both ground and airborne systems. The energy source for natural EM fields in the 10 Hz to 20 kHz audio-magnetotelluric (AMT) range is mainly worldwide thunderstorm activity. The usable range of AMT signals for a moving platform is between about 10 Hz and 500 Hz, which typically provides a depth of exploration from 100 m to potentially several kilometers. The remaining spectrum of the natural EM fields is not usable for the following reasons. There is a natural dead zone in the AMT spectrum, between 1 kHz and 5 kHz and for the 5 to 20 kHz range, although detectable, the AMT signals suffer from extreme variability and generally require stationary receivers.
Existing geophysical exploring systems use an aircraft to tow the FDEM systems and ground based systems for measuring the AMT fields. However, such a system is cumbersome and expensive, and thus, there is a need to have a new system that is capable of measuring both controlled EM fields and AMT fields at the same time eliminating inaccuracies associated with combining independent data.